Makefile: add clean-workdir target

This means it's easier to run e2e tests locally with a clean
starting point e.g make clean-workdir test-e2e-sharded-minimal

Makefile

@@ -382,10 +382,15 @@ verify-modules: modules  ## Verify go modules are up to date
 	hack/verify-go-modules.sh
 
 .PHONY: clean
-clean:
+clean: clean-workdir
 	rm -fr $(TOOLS_DIR)
 	rm -f $(GOBIN_DIR)/*
 
+.PHONY: clean-workdir
+clean-workdir: WORK_DIR ?= .
+clean-workdir:
+	rm -fr $(WORK_DIR)/.kcp*
+
 .PHONY: help
 help: ## Show this help.
 	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'